We can live without the ValidatorFactory so long as the user is not requesting DB constraints for the validations.

This sounds like a blocker if the user cannot create constraints on the entity fields (e.g. @NotNull).

DataSource is a tough one because of how we changed metamodel to leverage database-specific info during binding. Just need to make sure that metamodel binding works ok if a JDBC Connection is not available.

It would be helpful if you could somehow classify the new changes that wouldn't work without the database-specific info (performance enhancements?)

The idea would be that the EntityManagerFactoryBuilder would register an instance of this with the service registry or SessionFactoryBuilder.

Are you thinking that the thrown NotYetAccessibleException exception is ignored and initialization continues after skipping (early) use of the BM/VF/DS?